A general cylindrical roller bearing comprises an inner ring having a track surface around its outer periphery, an outer ring having a track surface around its inner periphery, a plurality of cylindrical rollers arranged between the track surface of the inner ring and the track surface of the outer ring so that they can roll freely, and a retainer retaining the cylindrical rollers at a predetermined intervals in a circumferential direction.
For example, when the inner ring comprises flanges at both ends, a relief groove is provided at the corner in which the flange surface of the flange and the track surface of the inner ring cross. This relief groove is provided as a relief region when the track surface and the flange surface are ground. In addition, a chamfer is provided at the corner part in which the rolling surface and the end surface of the cylindrical roller cross. The axial dimension between the flange surfaces opposed to each other in an axis direction is set a little larger than the length of the cylindrical roller, whereby a guide clearance is provided between the cylindrical roller and the flange.
According to the cylindrical roller bearing described above, since the rolling surface of the cylindrical roller and the track surface of the track ring are linearly in contact with each other, it has a high load facility of a radial load and it is suitable for high-speed rotation, but a heating value is great at the time of high-speed rotation as compared with a ball bearing, and it has a problem in that a lot of heat and abrasion are likely to be generated at the sliding contact part between the cylindrical roller and the flange, especially. More specifically, the cylindrical roller has a degree of freedom for inclination by the above-described guide clearance, so that it is inevitable that the axis line of the cylindrical roller is inclined with respect to the axis line of the bearing, that is, a skew is generated at the time of rotation of the bearing. When the cylindrical roller is skewed, an axial component is generated in the driving force applied from the track surface of the rotating side, and it becomes axial thrust force F that presses the end of the cylindrical roller to one flange. Thus, the friction resistance at the sliding contact part between the cylindrical roller and the flange surface is increased, which causes heat generation and abrasion in some cases.
Various kinds of improvements have been proposed for the above problems. For example, according to Japanese Patent Publication No. 58-43609, the height of a relief groove is made larger than that of a chamfer of a cylindrical roller, and a tapered surface extending to the outside in an axial direction at a predetermined angle is provided in a flange surface, to improve the lubricant state of the above sliding contact part.
In addition, according to Japanese Unexamined Patent Publication No. 7-12119, the outer peripheral part of the end surface of a cylindrical roller comes into contact with a flange surface at a part on the base end side shifted from the top end of the flange surface when the cylindrical roller is skewed, to reduce the edge load at the above sliding contact part as compared with a case where the outer peripheral part of the end face of the cylindrical roller comes into contact with the top end of the flange surface.
As described above, since the cylindrical roller has the degree of freedom for inclination by the guide clearance, while the cylindrical roller rotating and revolving at the time of the bearing rotation, it constantly varying its posture within a maximum skew angle. As schematically shown in FIG. 1, when a cylindrical roller 1 is skewed at a skew angle θ smaller than a maximum skew angle θmax, the cylindrical roller 1 is pressed to one side in the axial direction by the axial thrust force F and guided to roll in the state in which it is pressed to one flange surface 2a of an inner ring 2. At this time, the contact state between the cylindrical roller 1 and the flange surface 2a varies with the skew angle θ as follows.
That is, when the skew angle θ is in a range 0<θ<θ1, a boundary B1 between an end surface 1a and a chamfer 1b of the cylindrical roller 1 is in contact with a boundary B2 between a flange surface 2b and a relief groove 2c as shown in FIG. 2 (contact point is shown by a black circle ●), and when the skew angle θ is in a range θ1<θ<θ2, the boundary B1 between the end surface 1a and the chamfer 1b of the cylindrical roller 1 is in contact with the flange surface 2b as shown in FIG. 3 (contact point is shown by a black circle ●). Thus, when the skew angle θ is about to become approximately 2θ, the boundary B1 between the end surface 1a and the chamfer 1b of the cylindrical roller 1 is in contact with a boundary B3 between the flange surface 2b and a flange surface chamfer 2d (not shown). Then, both ends of the cylindrical roller 1 come in contact with both flange surfaces 2a, respectively and the skew angle θ reaches the maximum skew angle θmax (not shown).
FIG. 4 shows the relation (solid line) between the skew angle θ of the cylindrical roller 1 and the contact surface pressure P between the cylindrical roller 1 and the flange 2a, and the relation (dotted line) between the skew angle θ and the axial thrust force F applied to the cylindrical roller 1. As shown in FIG. 4, the axial thrust force F is increased as the skew angle θ is increased.
In the range 0<θ<θ1, the contact surface pressure P is relatively steeply increased as the skew angle θ is increased. This is related to the fact that the cylindrical roller 1 and the flange 2a come into contact with each other at the boundary B1 and the boundary B2 (shown in FIG. 2), and the axial thrust force F is increased as the skew angle θ is increased. Especially, in the range θ0<θ<θ1 (region shown by crossed hatching in FIG. 4), it has been confirmed from a test that the contact surface pressure P becomes a surface pressure level P0 or more in which the contact part is abraded.
When the skew angle becomes more than θ1, the contact surface pressure P is reduced to the surface pressure level P0 or less, and it makes a stable shift at a relatively low value although the skew angle θ is increased. This means that the contact state between the cylindrical roller 1 and the flange 2a is shifted from the contact between the boundary B1 and the boundary B2 (shown in FIG. 2) to the contact between the boundary B1 and the flange surface 2b (shown in FIG. 3).
When the skew angle θ becomes approximately θ2, the contact surface pressure P is rapidly increased again and when the skew angle θ becomes θ2, it becomes the surface pressure level P0 or more. This means that the contact state between the cylindrical roller 1 and the flange 2a is shifted from the contact between the boundary B1 and the flange surface 2b (shown in FIG. 3) to the contact state between the boundary B1 and the boundary B3.
As described above, the contact surface pressure P between the cylindrical roller and the flange becomes the surface pressure level P0 or more in which the contact part is abraded before the skew angle θ becomes the maximum skew angle θmax, that is, in the range θ0<θ<θ1 and θ2<θ<θmax, which is attributed to the major factor of the heat generation and abrasion at the contact part.
However, according to the above Japanese Patent Publication No. 58-43609, there is no recognition of the above phenomenon, so that its measures are not proposed. Furthermore, according to the above Japanese Unexamined Patent Publication No. 7-12119, although the contact state between the outer peripheral part of both end surfaces of the cylindrical roller and the flange surface at the maximum skew angle θmax is regulated, there is no recognition of the above phenomenon generated at the stage before the skew angle θ reaches the maximum skew angle θmax, so that its measures are not proposed also.
The same applicant of this application has proposed a cylindrical roller bearing suitable for the higher-speed rotation in Japanese Unexamined Patent Publication No. 2003-278745. The invention disclosed in the above document is characterized in that a critical skew angle θ1 that is a maximum skew angle in which the boundary between the end surface of a cylindrical roller and a chamfer comes into contact with the boundary between a flange surface and a relief groove is regulated to a predetermined angle or less. Thus, the contact state between the cylindrical roller and the flange is shifted from the contact between the boundaries (shown in FIG. 2), to the contact between the boundary and the flange surface (shown in FIG.  3) at a smaller skew angle, so that a contact surface pressure can be reduced.
The above Japanese Unexamined Patent Publication No. 2003-278745 will be described in detail hereinafter. As shown in an enlarged view in FIG. 5, a relief groove 2c is provided at a corner in which a flange surface 2b of each flange 2a and a track surface 2e of an inner ring 2 cross. The relief groove 2c is provided as a relief region when the track surface 2e and the flange surface 2b are ground mainly. The flange surface 2b is tapered such that it is gradually opened in an outer diameter direction, and a chamfer 2d is provided at a corner part in which the flange surface 2b and an outer diameter surface 2f of the flange 2a cross. Furthermore, a chamfer 1b is provided at the corner part in which a rolling surface 1c and an end surface la of the cylindrical roller 1 cross. Furthermore, the axial dimension between the flange surfaces 2b opposed to each other in an axial direction is made a little larger than the length of the cylindrical roller 1, so that a guide clearance S is provided between the end surface 1a of the cylindrical roller 1 and the flange surface 2b. 
A height “H” of the relief groove 2c from the track surface 2e of the inner ring 2 is set to be larger than a height “h” of the chamfer 1b from the rolling surface 1c of the cylindrical roller 1. Thus, a difference δ(δ=H−h) between the height “H” of the relief groove 2c and the height “h” of the chamfer 1b of the cylindrical roller is regulated to a predetermined value or less, so that the above-described critical skew angle θ1 can be regulated to the predetermined angle or less.
In addition, the height “H” of the relief groove 2c is the dimension from the track surface 2e to the boundary B2 between the relief groove 2c and the flange surface 2b in a radius direction. In addition, the height “h” of the chamfer of the cylindrical roller is the dimension from the boundary B4 between the rolling surface 1c and the chamfer 1b to the boundary B1 between the chamfer 1b and the end surface 1a in the radius direction.
FIG. 6 shows the relation (solid line) between the skew angle θ of the cylindrical roller 1, and the contact surface pressure P between the cylindrical roller 1 and the flange surface 2a, and the relation (dotted line) between the skew angle θ and the axial thrust force F applied to the cylindrical roller 1 in the cylindrical roller bearing disclosed in the above Japanese Unexamined Patent Publication No. 2003-278745. Although the contact surface pressure P is steeply increased as the skew angle θ is increased in a range 0<θ<θ1, since the critical skew angle θ1 is regulated to the small angle as compared with that shown in FIG. 4, the contact surface pressure P shifts at a value lower than the surface pressure level P0 in which the contact part is abraded (there is no region shown by the hatching in FIG. 4). More specifically, even when the cylindrical roller 1 and the flange 2a come in contact with each other at the boundary B1 and the boundary B2 (state shown in FIG. 2), as long as the skew angle θ is small, the axial thrust force F pressing the cylindrical roller 1 toward the flange 2a is small, so that the contact surface pressure P is relatively small.
In a range θ1<θ<θ2, similar to that shown in FIG. 4, the contact surface pressure P makes a stable shift at a relatively low value although the skew angle θ is increased. When the skew angle θ becomes approximately θ2, the contact surface pressure P is steeply increased again and it becomes the surface pressure level P0 or more after the skew angle θ reaches θ2. However, since the maximum skew angle θmax is regulated to the small angle, the angle range (θ2<θ<θmax) in which the contact surface pressure P exceeds the surface pressure level P0 is narrow.
As described above, the contact surface pressure P is reduced by regulating the critical skew angle θ1 to the small angle and shifting the contact state between the cylindrical roller 1 and the flange 2a from the contact state (shown in FIG. 2) between the boundary B1 and the boundary B2 to the contact state (shown in FIG. 3) between the boundary B1 and the flange 2b at a smaller skew angle, so that the heat generation and the abrasion can be prevented from being generated at the contact part.
According to the cylindrical roller bearing disclosed in the above Japanese Unexamined Patent Publication No. 2003-278745, although the heat generation and abrasion at the contact part between the cylindrical roller and the flange can be prevented to some extent, there is a room for improvement. That is, according to the above Japanese Unexamined Patent Publication No. 2003-278745, there is no consideration of reduction in the contact surface pressure (refer to FIG. 6) in the range of θ1 to θ2. For example, the cylindrical roller bearing for the planet gear in the wind power generation speed-up gear rotates at high speed in a highly-loaded state. In the case of the cylindrical roller bearing used in such highly-loaded and rotating at high speed, it is desirable that the heat generation and abrasion at the contact part is to be further prevented by further reducing the contact surface pressure in the range of θ1 to θ2 in which the contact surface pressure is relatively low.
In addition, as shown in FIG. 6, the first peak of the contact surface pressure is generated by the contact between the boundary B1 (boundary between the end surface and the chamfer of the cylindrical roller) at the upper end of the chamfer of the cylindrical roller, and the boundary B2 (boundary between the relief groove and the flange surface) at the upper end of the relief groove of the track ring. Preferably, the first peak of the contact surface pressure is to be further reduced.